Mitochondria are subcellular organelles present in all oxygen-utilizing organisms in which energy in the form of adenosine triphosphate (ATP) is generated, and oxygen in reduced to water. Ninety percent of the oxygen taken in is consumed in mitochondria. A substantial byproduct of this ATP generation is the formation of potentially toxic oxygen radicals. For example, it is estimated that 1-2% of all reduced oxygen yields superoxide (O.sub.2 -) and hydrogen peroxide (H.sub.2 O.sub.2). Other reactive oxygen species (ROS) that form are singlet oxygen (.sup.1 O.sub.2) and hydroxyl radical (OH.cndot.). Under stress conditions in the cell this can rise to 10% of all consumed oxygen. Mitochondrial membranes are sensitive to lipid peroxidation and depolarization resulting from these ROS. Mitochondrial damage is also a result of exposure to sunlight, which forms ROS as indicated above. Because damage to mitochondria is believed to be the cause or an important factor in some diseases, such as cancer, diabetes, cataract, neurodegenerative disease, porphyrias, cardiovascular disease, and also a contributor to the complications of aging, a method of protecting mitochondria from such damage, repairing such damage, is desired. Cellular damage from burns to the skin and lungs from contact with or exposure to fire and other sources of intense heat is mediated through radical damage. Furthermore, exposure to adverse environmental factors, including industrial air pollutants and petroleum and tobacco combustion products, may contribute to oxidative damage to pulmonary and other tissues of the body. In addition, various therapeutic regimens such as chemotherapeutic drugs and radiation therapy for the treatment of dysproliferative diseases induce significant oxidant-stress-related side effects, such as cardiotoxicity. The present invention relates to applied agents which protect the mitochondria from such damage.
L-ergothioneine is a sulphur-containing amino acid which is found in many mammalian tissues but is not endogenously synthesized and must be consumed in the diet. Although it exists in some tissues in millimolar quantities, its exact role is uncertain (see: Melville, 1959, Vitamins and Hormones 7:155-204). It is generally regarded as an antioxidant, although results are conflicting. Some regard it as a scavenger of hydrogen peroxide (see: Hartman, 1990, Methods in Enzymology 186:310-318), while others contend that it does not readily react with hydrogen peroxide but does scavenge hydroxyl radical (see: Akamnu et al., 1991, Arch. Biochem. Biophys. 298:10-16, 1991). Although previous in vitro studies have demonstrated its ability to protect DNA and proteins against phototoxic drug binding induced by UV radiation (e.g., van den Broeke et al., 1993, J. Photochem. Photobiol. B 17:279-286), and to protect bacteriophage against gamma-irradiation (Hartman et al., 1988, Radiation Research 114:319-330), in vivo results have not been as promising. Although L-ergothioneine has been claimed as useful in topical formulations for scavenging radicals and UV light protectants for hair and skin damage (e.g., WO 9404129), Van den Broeke et al. (1993, Int. J. Radiat. Biol. 63:493-500) did not find topically-applied L-ergothioneine effective in an animal model of UV-induced phototoxic drug binding to epidermal biomolecules. Other proposed in vivo uses have included lowering of circulating lipoprotein (a) levels (U.S. Pat. No. 5,272,166), and inhibiting skin pigmentation, for example, to remove dark spots and freckles (JP 63008335 and JP 61155302).
As described above, numerous disease processes are attributed to the body's adverse reaction to the presence of elevated levels of reactive oxygen species (ROS) described above. In the eye, cataract, macular degeneration and degenerative retinal damage are attributed to ROS. Among other organs and their ROS-related diseases include: lung cancer induced by tobacco combustion products and asbestos; accelerated aging and its manifestations, including skin damage; atherosclerosis; ischemia and reperfusion injury, diseases of the nervous system such as Parkinson disease, Alzheimer disease, muscular dystrophy, multiple sclerosis; other lung diseases including emphysema and bronchopulmonary dysphasia; iron overload diseases such as hemochromatosis and thalassemia; pancreatitis; diabetes; renal diseases including autoimmune nephrotic syndrome and heavy metal-induced nephrotoxicity; and radiation injuries. Certain anti-neoplastic drugs such as adriamycin and bleomycin induce severe oxidative damage, especially to the heart, limiting the patient's exposure to the drug. Redox-active metals such as iron induce oxidative damage to tissues; industrial chemicals and ethanol, by exposure and consumption, induce an array of oxidative damage-related injuries, such as cardiomyopathy and liver damage. Airborne industrial and petrochemical-based pollutants, such as ozone, nitric oxide, radioactive particulates, and halogenated hydrocarbons, induce oxidative damage to the lungs, gastrointestinal tract, and other organs. Radiation poisoning from industrial sources, including leaks from nuclear reactors and exposure to nuclear weapons, are other sources of radiation and radical damage. Other routes of exposure may occur from living or working in proximity to sources of electromagnetic radiation, such as electric power plants and high-voltage power lines, x-ray machines, particle accelerators, radar antennas, radio antennas, and the like, as well as using electronic products and gadgets which emit electromagnetic radiation such as cellular telephones, and television and computer monitors. Protecting mitochondria from these many etiologic agents is desirable.